The popularity of wearable and/or portable electronic devices has created a substantial market for such devices. Portable electronic devices include personal electronic devices, such as smart phone, cell phones, MP3 players, and Bluetooth, etc. One limitation of such devices is the amount of energy that can be conveniently stored in the devices. Accordingly, substantial resources have been devoted to maximizing the energy storage capacity for both a given volume and a given weight. Nonetheless, portable electronic devices are still limited by the amount of energy that can be stored in the devices.
Consequently, portable electronic devices require frequent recharging. Moreover, as the device ages, the capacity of the energy storage system of the device deteriorates, necessitating more frequent charging.
Recharging a portable electronic device is generally a simple matter. A number of convenience enhancing devices have been developed allowing portable electronic devices to be rapidly charged and to be charged using a variety of power sources such as 12 v power systems commonly found in motorized vehicles. Additionally, backup batteries are commonly made available so that a fresh battery can be used to replace a depleted battery.
Even with all of the advances in powering portable electronic devices, however, providing power can be a challenge. For example, many people enjoy using portable electronic devices while hiking. The availability of power sources for recharging portable electronic devices is very limited, however, along hiking trails. Even when charging sources are available, however, recharging the power system of the portable electronic device requires the portable electronic device or the power source to remain in a specific location. Even for quick charging systems, the delay in activities is an undesired consequence.
In response to the foregoing limitations, the possibility of scavenging human power and either using it directly, or storing it for later use, to power portable electronic devices has been explored. Power harvesting generators which use human motion offer an attractive grid-free and portable energy source that can be used to power and recharge wearable and personal electronics. These generators harvest energy from everyday human motion, such as walking, running, standing up, and sitting down and use the harvested energy to charge the battery (or other storage reservoir) of a personal electronic device or even power the electronic devices directly.
In general, power harvesting devices are mechanical-to-electrical energy converters that usually consist of a mass-spring system coupled to a frame which is displaced by outside vibrations, shocks, or other motion. The mass-spring system acts as a damper for the motion of the frame, thereby acquiring kinetic energy. Transduction of mechanical to electrical energy by mass-spring system can be electromagnetic (magnet moving relative to a coil), electrostatic (charged objects moving past each other), or piezoelectric (strain in a bending element produces output voltage).
Transduction of human motion for powering wearable or portable electronics presents particular challenges. By way of example, frequencies of ordinary human motion (e.g. walking) are typically very low (˜1-2 Hz), the amplitudes of the movements are high (˜10 cm), and the weight and size of the device is limited to unobtrusive dimensions. As a consequence, the amount of power available from typical generating systems is limited to a few mW. Moreover, wearable electronics are becoming increasingly sophisticated and consuming more and more power.
Another limitation of known systems is that the known systems harvest power in only one dimension. By way of example, a moving piston within a generator positioned in the heel of a shoe can be used to generate power. Of course, any energy available from motion in other directions, such as pivoting motions, is lost.
Accordingly, there is a need for a lightweight generator that can be used to convert a movement into power. It would be beneficial if such a device were not limited to harvesting power available in a single dimension.